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A Sweet Recipe for Baby Boys

Some parents will try anything to influence the sex of their child. Eat meat if you want a boy, some say; fish and vegetables for a girl. Now, a new mouse study indicates that the belief that a mother's diet can influence a baby's sex is not so far-fetched. By altering a female mouse's blood-glucose levels around conception time, researchers skewed the sex ratio of the resulting offspring.

In the 1970s, biologists Robert Trivers and Dan Willard proposed that pregnant animals invest more in offspring that will give them the largest number of grandchildren. For species in which males mate with more than one female, healthy mothers should have more boys, whereas less healthy mothers should have more girls. Although weak female offspring still find someone to mate with, weak males may never mate at all. Previous research has shown an association between more male offspring and a spike in blood sugar due to stress or food abundance (ScienceNOW, 26 June 2006), and embryos grown in the lab become male-biased as they develop, but a causal link between sugar and sex ratio in a living animal has not been proven.

Elissa Cameron, a behavioral ecologist at the University of Pretoria in South Africa, decided to test the diet theory in mice. She and colleagues added a steroid called dexamethasone, which is known to decrease blood glucose levels, to the drinking water of 20 female mice, leaving 20 others alone. Then they introduced males for mating and removed them after 3 days. The team also took blood samples to determine blood-sugar levels before and after the dexamethasone treatment.

If the Trivers and Willard theory holds, "you would expect mice with higher glucose to give birth to more sons," says Cameron, because more sugar would indicate more abundant food and a healthier mother. And that is what the team found. The mice that received the steroid saw their glucose levels drop compared to untreated mice, and they gave birth to sons 42% of the time versus 54% of the time for sugar-rich mothers, Cameron's team reports this week in the Proceedings of the Royal Society B.

Biomedical scientist Cheryl Rosenfeld of the University of Missouri, Columbia, says she is surprised to see such a dramatic effect. The work adds important evidence to the diet/sex-ratio theory, says behavioral ecologist Sven Krackow of Humboldt University in Berlin, although he notes that there is still no explanation for the effect.

Cameron says the next step is to understand how blood-glucose levels affect glucose levels in the uterus. In addition to helping biologists better understand parental strategies, the findings could have implications for the livestock industry, she notes, in which females are preferred over males because they produce milk and babies.